Thermoplastic elastomeric block copolymers differ in molecular structure from typical plastic and commercial rubbers, which are generally homopolymers or random copolymers. That is, thermoplastic elastomeric block copolymers generally comprise two incompatible polymers, a thermoplastic end block polymer, typically polystyrene, chemically joined with one of several elastomeric mid block polymers. In use, the block copolymers tend to provide an elastic lattice structure interconnected by domains formed by their thermoplastic end blocks. Since the lattice structure is the result of physical rather than chemical forces, it may be destroyed either by dissolving the copolymer in a solvent or by heating it beyond the glass transition temperature of its thermoplastic end blocks. Upon evaporation of the solvent or cooling below the glass transition temperature of its thermoplastic end blocks, a structure may be re-imparted to the block copolymer. Such block copolymers are thus recyclable.
Thermoplastic block copolymers can include styrene-butadiene-styrene copolymers (SBS), styrene-isoprene-styrene copolymers (SIS) and styrene-ethylene/butylene-styrene copolymers (SEBS). In addition to the traditional ABA-type tri-block polymers, such copolymers are available in the radial (A-B).sub.n and a di-block (A-B) structures. Prior to processing, the polystyrene end blocks of such copolymers are associated in rigid domains through physical cross-linking to yield a continuous three dimensional network. During processing in the presence of heat and shear or solvent, the polystyrene domains soften and permit flow and after cooling, reform to lock the interconnecting elastomeric network in place. The styrene domains can impart high tensile strength to the resulting structure and the elastomeric mid block polymers can impart elasticity, cold flow flexibility and fatigue resistance.
Thermoplastic, non-vulcanite elastomeric copolymers have been developed for use in a wide variety of applications, such as gaskets, seals, blown films, elastic fibers, hot melt adhesives, sealants, caulking and tapes.
It is their distinctive molecular structure that is the key to the wide utility of thermoplastic elastomers. However, the structural makeup of thermoplastic elastomers that gives them such a wide range of capabilities also contains the seeds of weakness. Specifically, the elastomeric mid block that imparts flexibility to the structure also decreases the compound's abrasion resistance, life, strength, and chemical inertness. In addition, while the plastic end blocks add strength to the structure, they also limit the amount of elastic deformation that the structure can tolerate. For instance, when the thermoplastic end block is heated, it melts and flows. Since the thermoplastic end block has no "memory", it retains its new configuration when it cools. That is, the deformation caused by the flowing of the end block is irreversible. The structure of the thermoplastic elastomeric copolymer is also highly susceptible to degradation by hydrocarbons and solvents. While such degradation is useful in a recycling sense, it is unacceptable in other applications.
A composition that can be added to the thermoplastic elastomeric copolymers to increase the mechanical properties such as abrasion resistance, solvent resistance, useful life, strength, and the like would be welcome by users of thermoplastic elastomeric copolymers.
U.S. Pat. Nos. 4,596,839 and 4,962,136 disclose the improvement of the elastomer compositions by an additive including particulate polytetrafluoroethylene (PTFE) and an amount of particulate molybdenum disulfide (MoS.sub.2) effective to provide uniform mixing of the polytetrafluoroethylene and the elastomer composition. These patents further disclose a new composition can comprise about 25 to about 80 percent PTFE and about 1 to about 30 percent MoS.sub.2 by weight, with the balance of elastomer. The patents also disclose preferable compositions including about 2 to about 6 percent of PTFE that is fibrillatable and fibrillated in the composition when combined with an effective amount of MoS.sub.2. The polytetrafluoroethylenemolybdenum disulfide additives are disclosed as being useful in polymers, generally known as rubbers, including natural rubber and synthetic rubber elastomers and other polymers capable of forming elastic solids with similar properties. More specifically, such elastomers include, in addition to natural rubber, styrene butadiene rubber, acrylonitrile butadiene rubber, butyl rubber, ethylene-propylene rubber, polyurethane elastomers, CIS-polybutadiene polychloriprene, poly(epichlorohydrin), polyacrylate, silicone rubbers, poly(fluorinated hydrocarbons), olefin polysulfide, polyisoprene and the like. It is also disclosed that such compositions can also include plasticizers and softeners, extenders, reclaimed rubber, fillers, reinforcing fillers, coloring agents, antioxidants, accelerators and vulcanizing actuators.
Whenever composition ingredients are expressed in percentages, it is to be understood that the expressed percentage is the percent by weight of the resulting composition, unless otherwise stated. Where compositions are expressed in parts, it is to be understood that they are expressed in parts per hundred rubber by weight.